A trailer coupling of this type is known from EP 1 796 926 B1, in which the angle measuring device comprises a permanent magnet and sensors that are sensitive to a magnetic field. In this case, however, components of the angle measuring device are provided both on the coupling ball and on the coupling ball holder. Thus, when a trailer is used which has a simple coupling ball holder in which no components of the angle measuring device are additionally built in, the rotation detection does not work.